Annealing and operating temperatures effect on spray-deposited nanocrystalline ZnO thin-film gas sensor

In present research, nanostructure ZnO thin films have been fabricated by chemical spray pyrolysis method on glass substrates at temperature of 450 °C and annealed at different temperatures (400, 500, and 600) °C. Crystal structure results showed that all prepared ZnO thin films are polycrystalline structures in nature and hexagonal wurtzite phase, and the preferential orientation is along (002) plane. The annealed films showed appear of new peaks and variations in the intensity of the preferential orientation. The crystallite size was found to be decreased, the microstrain was increased and the stress was negative (compressive) and decrement with annealing temperature. Surface texture results showed uniform granular surface morphology for all samples and the surface roughness increased from 3.48 to 9.75 nm for as-deposited and annealing temperature at 400 °C, and then, it decreases at annealing temperatures (500 and 600) °C. The fabricated nanocrystalline ZnO gas sensors were investigated at a different mixing ratio of NO2 gas (5, 10, 15, 20, 25, 30, and 35) % and at different operating temperatures (R.T, 100, 200, and 300) °C using bias voltage of (10 Volt). Annealed nanocrystalline ZnO sensors exhibit a decrease of resistance when exposed to NO2 gas and showed a very high sensitivity for NO2 gas that can be accomplished at the annealing temperature of 500 °C. With the increase of NO2 gas concentration, ZnO thin films exhibit an increase in the sensitivity. The optimal operating temperature obtained about 200 °C for all samples. The maximum sensitivity is (240%) with a fast response time (0.3 s) and the highest recovery times are about 9.4 s which were achieved at operating temperature 200 °C and annealing temperature 500 °C. The novel result we obtained is the negative effect of annealing temperature on the structural and the surface topography which produced a nanocrystals with a high surface area which is very beneficial and resulted in increasing the sensitivity of the sensor.